(a) Field of the Invention
This invention relates to a flow velocity measurement processor for a laser Doppler velocimeter which utilizes the interference of a laser beam to measure, for example, a flow velocity. More particularly, this invention relates to a signal processing system for Doppler burst signals.
(b) Description of the Prior Art
Generally, in laser Doppler velocimeters, a laser beam is divided at a beam splitter into two beams and focused with a lens. A particle, traveling through interference fringes at the measuring point, causes light scattering. The scattered light, by way of the focusing apparatus, is received at a photomultiplier wherein the scattered light is transformed into a Doppler burst signal. Then, to obtain a flow velocity of the particle, the Doppler burst signal is processed by a signal processing system.
Conventionally, counters or trackers are used for this signal processing system. Counters measure the number of periods of an input frequency with a built-in clock and then calculate its reciprocal number to obtain a Doppler frequency. On the other hand, trackers use a Phase-Locked-Loop(PLL) by which an input frequency and a frequency of a built-in voltage controlled oscillator(VCO) are kept at a constant level. Thus, a Doppler frequency is obtained from this tracking frequency. As the Doppler frequency is proportional to the velocity of the particle, both counters and trackers calculate the flow velocity from the Doppler frequency.
The above-mentioned Doppler burst signal consists of various kinds of noise such as shot noise or white noise. This means that the laser Doppler velocimeters employing counters or trackers for the signal processing system can not measure the flow velocity from the Doppler signal with low signal-to-noise ratio(SNR). In other words, there is a limitation of the measurable range for this type of laser Doppler velocimeter. Furthermore, both counters and trackers have another disadvantage; in counters, time dependent data required to obtain vortex scales in a turbulent flow are not available, while the analog processing employed in trackers makes its application for measurements in a high turbulent flow with low particle density impossible.